1. Field of the Invention
The present invention relates to a develop roller and a develop unit used in a copier, a facsimile machine, or a printer to deliver developer on a develop sleeve to a develop area between a photoreceptor drum and a develop sleeve, develop an electrostatic latent image on the photoreceptor drum, and generate a toner image as well as to a process cartridge and an image forming apparatus including such a develop unit.
2. Description of the Prior Art
Japanese Patent Application Publication No. 2003-255692 (Reference 1), No. 2004-191835 (Reference 2), and No. 2007-86091 (Reference 3) disclose a develop sleeve whose surface is sandblasted, grooved or processed by electromagnetic blasting in order to surely deliver developer to a photoreceptor drum.
Being sandblasted or grooved, the develop sleeve in high-speed rotation is prevented from slipping and retaining the developer, preventing a decrease in image density.
The develop sleeve can be made of any of aluminum alloy, brass, stainless steel and conductive resin. It is mostly made of aluminum alloy in terms of cost efficiency and workability. In sandblasting, an aluminum tube is extruded into a sleeve shape at high temperature and sprayed with abrasive grains under ambient temperature, thereby forming unevenness on the surface at about a roughness Rz5.0 to 15 μm, for example. The sandblasted develop sleeve can prevent slippage of developer owing to the unevenness on the surface even during high-speed rotation.
However, there is a problem with the sandblasted develop sleeve in terms of durability since the unevenness on the surface is extremely fine so that it is abraded and the surface is gone smooth as the number of prints increases with time. Accordingly, amount of developer the develop sleeve delivers decreases gradually, weakening the color of generated images. The develop sleeve can be made of a high hardness stainless steel or subjected to hardening on the surface. However, this is not desirable because of an increase in manufacture costs.
To form grooves on the surface of the develop sleeve of aluminum alloy, for example, an aluminum tube is extruded into a sleeve shape at high temperature, extracted under ambient temperature, and cut with a die. The cross-sectional shapes of grooves are generally square, V-form, or U-form, the depth thereof is about 0.2 mm from the surface and the number thereof is about 50 for a develop sleeve in outer diameter of φ25. The develop sleeve with the grooves can prevent slippage of developer even in high-speed rotation.
Moreover, the grooves are much larger than the unevenness formed by sandblasting and not abraded with time and do not cause a decrease in delivery amount of developer. The develop sleeve with the grooves are less abraded in long-time use than the sandblasted develop sleeve and can stably deliver developer.
However, it has a problem with this develop sleeve that image density may periodically vary or uneven pitch may occur because of a difference in delivery amount of developer between the grooves and non-groove portions. Generally, the deeper the grooves, the better the developer delivery performance but the more likely uneven pitch occurs due to a difference in develop field intensity of the grooves and the non-groove portions. With shallower grooves, toner, additives, or carrier in the developer is likely to get stuck in the grooves, largely decreasing the developer delivery performance and amount of developer attracted. Insufficient delivery attraction is likely to cause uneven pitch.
In view of solving the above problems, the develop sleeve disclosed in Reference 1 comprises grooves in depth of 0.05 mm or more and 0.15 mm or less to prevent uneven pitch and maintain developer delivery performance. However, along with improvement in image reproducibility by advanced image forming technique such as adaption of toner or carrier of smaller particle size or proximity developing, the uneven pitch is more noticeable. For example, using toner in mean volume diameter of 8.5 μm or less, due to its good image reproducibility a variation in amount of developer or uneven pitch is conspicuous.
FIGS. 19 and 20 show a prior art developer sleeve attracting developer. In the drawings developer 203 slips and decreases in amount on portions without grooves in a develop area D between a develop sleeve 200 and a photoreceptor drum 201, causing a decrease in image density and uneven pitch. It is in general necessary to deliver a large amount of developer 203 to the develop area D to acquire sufficient image density.
The develop sleeve 200 is typically rotated 1.1 to 2.5 times faster than the photoreceptor drum 201. A friction between the developer 203 passing the develop area D at high speed and the photoreceptor drum 201 rotating at relatively low speed becomes a load resistance on non-groove portions of the surface of the develop sleeve 200. As shown in FIG. 19, slippage or insufficient attraction of the developer 203 occurs on the non-groove portions of the develop sleeve 200, so that in the develop area D the amount of developer the develop sleeve 200 holds differs between the downstream and the upstream in the rotary direction. The amount on the downstream side is smaller than that on the upstream side. Meanwhile, as shown in FIG. 20, there is no slippage or insufficient attraction of the developer 203 while the grooves are passing the develop area D. Thus, developer slippage occurs periodically due to the grooves passing the develop area D, which changes an amount of the developer 203 and results in uneven pitch in due to uneven image density.
An image forming apparatus disclosed in Reference 2 uses a developer of toner in mean volume diameter 4 μm or more 8.5 μm or less and includes a develop sleeve having grooves extending in a longitudinal direction and arranged with an interval smaller than a width of a photoreceptor drum in a develop area in a moving direction. In this image forming apparatus there is always at least one sleeve groove in the develop area to prevent slippage of the developer, makes it possible to reduce a variation in amount of the developer in the develop area. Thus, even with use of such a small particle size toner as 8.5 μm or less in mean volume diameter, the apparatus can generate high-quality images with good reproducibility and less uneven pitch due to uneven image density.
However, there is a problem with this develop sleeve that since the grooves are formed by drawing an aluminum tube with a dice by cold working and finished by cutting or grinding and need be disposed with a narrower interval, there may be an increase in deviation of the depth of the grooves. The deviation in the groove depth may cause unevenness in image density.
It is possible to reduce the length of the interval or reduce the deviation in the groove depth by cutting the grooves one by one or several at a time. However, it increases the number of processing steps and manufacture costs.
The develop sleeve formed by electromagnetic sandblasting disclosed in Reference 3 can reduce a decrease in delivery amount of developer due to a degradation with time. However, the surface of the develop sleeve is randomly hit with a linear material by sandblasting so that it is difficult to set a proper processing condition in order to maintain an optimum attraction amount of the developer and elongate the longetivity of the develop sleeve. It is also difficult to increase the attraction amount of developer in view of high-quality image generation with a higher-speed machine in the future.
Furthermore, a doctor blade is provided adjacent to the develop roller to constantly adjust the thickness of the developer on the develop roller. The toner supply amount to the photoreceptor drum is adjusted by a gap (hereinafter, doctor gap) between the doctor blade and the surface of the develop roller. Irrespective of the surface shape (surface processing) of the develop roller, the develop roller may be warped by a frictional resistance of developer passing the doctor gap and a magnetic attraction of the developer. This may cause the doctor gap in the longitudinal center of the develop roller to be widened beyond both ends of the develop roller. Accordingly, a problem arises that toner supply amount in the longitudinal center of the develop roller is larger than that in both of the end portions, causing unevenness in image density in the longitudinal direction of the develop roller.